September 16, 1997

Scientists Use Light to Create Particles

By MALCOLM W. BROWNE

trailblazing experiment at the Stanford Linear Accelerator
Center in California has confirmed a longstanding prediction by
theorists that light beams colliding with each other can goad the
empty vacuum into creating something out of nothing.

In a report published this month by the journal Physical Review
Letters, 20 physicists from four research institutions disclosed
that they had created two tiny specks of matter -- an electron and
its antimatter counterpart, a positron -- by colliding two
ultrapowerful beams of radiation.

The possibility of doing something like this was suggested in
1934 by two American physicists, Dr. Gregory Breit and Dr. John A.
Wheeler. But more than six decades would pass before any laboratory
could pump enough power into colliding beams of radiation to
conjure up matter from nothingness. The Stanford accelerator
finally provided enough energy to do it.

Dr. Adrian C. Melissinos of the University of Rochester, a
spokesman for the group, said in an interview that the weaker of
the two light beams used in the experiment was produced by a
trillion-watt green laser. That in itself fell far short of the
needed energy, even though the pulsed green laser is one of the
world's most powerful.

But the opposing beam of radiation was another story; boosted by
energy drawn from electrons whizzing down the two-mile-long
Stanford accelerator, this second beam of radiation was some 10
billion times as powerful as the green laser beam.

The paths of colliding electrons and photons in the experiment
were as complicated as those choreographed by an expert pool player
planning a difficult shot.

Photons of light from the green laser were allowed to collide
almost head-on with 47-billion-electronvolt electrons shot from the
Stanford particle accelerator. These collisions transferred some of
the electrons' energy to the photons they hit, boosting the photons
from green visible light to gamma-ray photons, and forcing the
freshly spawned gamma photons to recoil into the oncoming laser
beam. The violent collisions that ensued between the gamma photons
and the green laser photons created an enormous electromagnetic
field.

This field, Melissinos said, "was so high that the vacuum
within the experiment spontaneously broke down, creating real
particles of matter and antimatter."

This breakdown of the vacuum by an ultrastrong electromagnetic
field was hypothesized in 1950 by Dr. Julian S. Schwinger, who was
awarded a Nobel Prize in physics in 1965. The creation of matter by
colliding photons of radiation is believed to take place in some
stars, but it was never observed in laboratory experiments before,
largely because the required energy is beyond the reach of
conventional laboratory equipment.

With his special theory of relativity, Einstein showed that
matter and energy are equivalent and can be transmuted through the
equation E equals mc2; that is, energy in ergs is equal to mass in
grams times the speed of light squared, in centimeters per second.
This accounts for the vast energy released by small amounts of
matter in nuclear explosions, but it also means that staggering
amounts of energy are required to create even the tiniest particles
of matter.

The hardest part of the project, in which scientists employed by
the Stanford Linear Accelerator Center collaborated with colleagues
from the University of Tennessee in Knoxville, Princeton University
and the University of Rochester, was synchronizing the timing of
laser and electron pulses, Melissinos said. The green laser pulse,
traveling at the speed of light, was only one half millimeter long.
That pulse had to be timed to collide with an electron pulse almost
as it emerged from the two-mile-long beam line.

The experiment, Melissinos said, is unlikely to have many
practical applications, although it might help in the design of a
new generation of research accelerators. Existing accelerators use
particles of matter as projectiles -- protons, electrons or entire
atoms. But a possible future accelerator that physicists call a
"gamma-gamma machine" might work by colliding opposing beams of
photons, especially gamma-ray photons.

Meanwhile, Melissinos and his colleagues expect to use photon
collisions as a way to explore the intricacies of quantum
electrodynamics -- a highly successful but complex theory explaining
the interactions of electromagnetic fields with matter.